THIS invention relates to electronic radio frequency (RF) identification systems comprising a reader and a plurality of transponders. The invention more particularly relates to transponders forming part of such a system.
Known electronic systems of the aforementioned kind comprise an interrogator or reader comprising a transmitter for transmitting an energizing or interrogation signal to the transponders and a receiver for receiving a response signal from the transponders. A microprocessor in the reader identifies a particular transponder by a data stream in the response signal. Each transponder comprises an antenna connected to an integrated circuit hosting electronic circuitry for receiving and collecting power from the interrogation signal to present a high enough voltage on a voltage storage capacitor, to power the circuitry which in turn generate the aforementioned data stream. The data stream is used by the transponder to modulate the energizing signal and to reflect back to the reader a portion of the energy in the energizing signal, by what is known as backscatter modulation. In the applicant's U.S. Pat. No. 6,054,925, there is disclosed a transponder with a high input impedance which the applicant believes will improve on the limited effective distance of prior art systems. The higher the input impedance of the integrated circuit and the antenna feedpoint impedance, the higher the voltage recovered from the energizing signal and stored on the capacitor.
It is also known that the higher the aforementioned impedances, the higher the quality factor (Q) of the circuit and consequently the narrower the bandwidth of the transponder becomes. Known transponders with relatively high impedances have a bandwidth of in the order of 1%-2%. The applicant has identified a need for a practical system to cover the range 860 MHz to 930 MHz and which hence requires a bandwidth of about 7% to 10%.
In U.S. Pat. No. 5,317,330 to Everett there is disclosed a two way communication system wherein a series or resonant circuit and a parallel or anti-resonant circuit are utilized to divide a bandwidth into a transmitting part and a separate receiving part. The result is two discrete narrow bands for use in transmitting and receiving coded information at different frequencies. Such a system would not be able to cover the aforementioned range adequately.